The most common anatomical pattern is a single sciatic nerve which passes anterior to the piriformis muscle, however it is important to note that there are other common anatomical variants.
Pokorny et al in a study of 91 cadavers found atypical anatomy of the sciatic nerve in almost 21% of cases. The normal anatomy seen in 79% is a single sciatic nerve exiting underneath (passing anterior) to the piriformis. The most common variant, seen in 14%, is the sciatic nerve splitting proximal to the piriformis with one branch passing anterior to and one branch passing through the piriformis.
Illustration A is an anatomic drawing of the sciatic nerve passing anterior to the piriformis.
Pokorný D, Jahoda D, Veigl D, Pinskerová V, Sosna A. Topographic variations of the relationship of the sciatic nerve and the piriformis muscle and its relevance to palsy after total hip arthroplasty. Surg Radiol Anat. 2006 Mar;28(1):88-91. Epub 2005 Nov 26.
PMID:16311716 (Link to Abstract)
Tornetta P III: Hip dislocations and fractures of the femoral head, in Bucholz RW, Heckman JD (eds): Rockwood and Green’s Fractures in Adults, ed 5. Philadelphia, PA, Lippincott Williams & Wilkins, 2001, p 1556